Victoria Beckham a UN AIDS ambassador

Designer and singer Victoria Beckham has taken on a new role as a UN ambassador on AIDS, saying she was transformed by a visit to South Africa.

The former Spice Girl made the announcement on Thursday at the briefing room of the United Nations headquarters, in a rare moment of celebrity glitz during an annual week dominated by back-to-back statements by world leaders and UN bureaucrats.

Beckham said she had long been interested in charity but had previously taken a supportive role in work led by her husband, British football icon David Beckham, as well as fellow singers Elton John and Annie Lennox.

But Beckham, who turned 40 this year and has four children, said she felt a new responsibility after travelling several months ago to Cape Town and meeting mothers living with HIV.

"South Africa was a huge turning point for me. And I don't know, why has it taken for me to get to 40 years old to realise, I need to stand up? I need to use what I have to make a difference," she said.

"For some reason, people will listen to what I have to say, so I'm going to speak on behalf of the incredible women" who have HIV and are caring for children, she said.

"I'm not going to sit here and pretend to know everything at this stage - I don't - but I want to learn. I want to help," she said next to a beaming Michel Sidibe, director of the UNAIDS agency.